1. Field of Invention
This invention relates to a system and method for monitoring and controlling the level of heavy metal contamination in the slurry of a soil or other particulate material washing process.
2. Background of the Invention
Contamination of soil and other particulate materials, such as for instance sludges, sediments, scrap yard dust and the like, is becoming a more common environmental problem. Often the particulate material is contaminated with heavy metals such as, for instance, cadmium, copper, lead, mercury radium, uranium and thorium.
Various methods and systems have been developed for reducing the concentration of these heavy metal contaminants in the particulate material to acceptable levels, typically well below 1000 ppm and generally below 100 ppm. One such technique is disclosed in commonly owned related patent application Ser. No. 07/529,092 filed May 25, 1990 and entitled "Method and Apparatus for Cleaning Contaminated Particulate Material." In this process, the soil or other particulate material is first washed with a leachate or surfactant to mobilize soluble and dispersible contaminants. Large particles are mechanically separated, washed and returned to the site. The fines in which the heavy metals are concentrated, are together with the leachate and solubilized contaminants, then separated from the intermediate sized particles by a counterflow of washwater to produce a waste slurry which is disposed of or further treated. The slurry of separated intermediate sized particles and wash water is dewatered to produce additional recovered particulate material. The size of the fines separated from the intermediate sized particles can be varied (more fines recovered with the intermediate sized particles or more diverted in the waste slurry) to adjust the level of contamination remaining in the recovered particulate material.
There is a need for a reliable on-line system and method for monitoring the concentration of the heavy metal contaminants in the recovered particulate material for assuring that the required reduction in contamination level has been realized and to control the process to achieve such a result.
One of the difficulties is that it is the residual concentration of the heavy metal contaminants in the recovered particulate matter which must meet the required standards, not the slurry. There is a further need therefore for such a system and method which can determine on-line the concentration for residual heavy metal contaminants in the solids fraction of a slurry.